The experiments described in this proposal are designed to provide a fuller characterisation of the mechanisms involved in the restriction of the expression of the viral genome in avian sarcoma virus-infected mammalian cells. The natural laboatory host cell for the avian sarcoma virus (ASV) is the chicken embryo fibroblast within which the virus is able to replicate and from which infectious progeny virus are produced. Transformation of the infected chicken embryo fibroblast appears to be highly efficient. Infection of a mammalian cell, such as a rat embryo fibroblast, however, establishes a virus-cell interaction which is non-permissive for full expression of ASV genome. Infectious progeny are not produced. Usually, however, the entire viral genome is present in these infected cells and its presence can be demonstrated by virus rescue through fusion with chicken cells. A number of clones of ASV-infected rat cells possess quite distinctive biological properties and suggest that the rat cell restricts the expression of the viral genome in different ways. The principal approaches to be used in these experiments will involve an analysis of the arrangement of viral DNA sequences within the cellular DNA, characterisation of the viral RNA transcribed from the viral DNA sequences and an identification of viral proteins synthesised in the infected cell.